1. Field of the Invention
The present invention relates to a lubrication coating composition which can form a dry coating having superior bonding properties to a base material such as a piston skirt and superior sliding properties, and more particularly to a double-layer lubrication coating composition having an upper-layer coating composition and a lower-layer coating composition.
2. Description of Related Art
Conventionally, a method for forming a lubrication coating on a surface of a sliding component of an internal combustion engine has been adopted as one of means for improving the wear resistance and seizing resistance of the relevant component. Then, there has been proposed a composition in which a solid lubricant is compounded into a resin acting as a binder as a means for improving the sliding property of the lubrication coating.
For example, Japanese Patent Unexamined Publication JP-A-01-261514 describes a sliding material which is compounded of 20 to 80% by volume of a polyimide resin or a polyamide-imide resin, 10 to 60% by volume of polytetrafluoroethylene as a solid lubricant, 0.5 to 20% by volume of alumina and the like.
In addition, Japanese Patent Unexamined Publication JP-A-64-087851 describes that a coating is applied to a piston by employing a solid film lubricant made up by compounding 25 to 125 parts by weight of polytetrafluoroethylene per 100 parts by weight of polyimide resin in order to increase the wear resistance of an engine component such as a piston.
Additionally, Japanese Patent Unexamined Publication JP-A-06-279708 describes a low-friction lubrication paint designed to form a coating having a low friction coefficient and superior wear resistance in which 5 to 300 parts by weight of a solid lubricant such as molybdenum disulfide, graphite, boron nitride or the like and 5 to 100 parts by weight of a modifier such as a vinyl resin, polybutadiene or the like are compounded per 100 parts by weight of a high-strength, heat-resistant binder such as an epoxy resin.
Furthermore, Japanese Patent Unexamined Publication JP-A-07-097517 describes a sliding resin composition designed to attain a low friction coefficient and high wear resistance which is made up of 50 to 73 wt % of at least one of a polyamide-imide resin and a polyimide resin, and, as solid lubricants, 3 to 15 wt % of polytetrafluoroethylene, 20 to 30 wt % of molybdenum disulfide and 2 to 8 wt % of graphite, a total sum of the solid lubricants being 27 to 50 wt %.
In order to reduce the friction coefficient of a lubrication coating in which a solid lubricant is compounded into a resin, the amount of polytetrafluoroethylene to be compounded needs to be increased. However, on the other hand, when the amount of polytetrafluoroethylene to be compounded is increased, since polytetrafluoroethylene and the heat-resistant resin have no bonding force, the wear amount of the resin coating layer is increased. Therefore, it has not been possible to achieve low friction coefficient and high wear resistance simultaneously.
In addition, it has been known that among the solid lubricants, when the amounts of molybdenum disulfide and graphite to be compounded are increased, seizing resistance is increased. However, in the event that molybdenum disulfide and graphite are compounded more than necessary, the strength of the resin coating layer itself is decreased extremely remarkably, and this tends to increase the wear of the resin coating layer.
In addition, it has been known that compounding, for example, alumina and/or silicone nitride as a hard filling material is effective to increase the wear resistance of the resin coating layer. However, it has also been known that in the event that the amount of alumina and/or silicone nitride to be compounded is increased, the friction coefficient on the surface of the resin coating is increased, while the seizing resistance thereof is decreased.